The present invention relates to orthopedic implants for use in repairing fractured bones or bones that have undergone a surgical procedure for therapeutic purposes, e.g., deformity correction, reconstruction, arthrodesis, etc. The implants described herein refer more specifically to an orthopedic bone fixation plate and screw construct. In situations where the bone fixation occurs near a joint and the surgeon desires to angle the screw to avoid encroachment into the joint, where the surgeon wishes to intentionally angle a screw to cross a fracture gap or joint in order to achieve fusion, or other surgical applications where intentional screw angulation is useful, it is desirable to insert the fixation screw at an angle offset from the central axis of the screw hole in the bone plate and an orientation directed radially about the circumference of the hole. In addition, there are clinical circumstances in which rigidity of fixation is desired, specifically between the screw and the plate, where it is desirable for forces acting on the bone to be counteracted in part or in whole by the screw-plate construct. In these situations, it is preferred to provide a means for rigidly locking the screw to the plate where the shaft of the screw is substantially fixed at various angles to the central axis and orientations about the circumference of the screw hole. This type of plate/screw construct may be referred to in orthopedics as variable-angle locking.
Variable angle locking in orthopedic applications may utilize various mechanical designs to secure the head of the bone screw to the plate hole. These designs may include cross-threading, self-tapping, and frictional “crush-lock” features that provide a degree of rigidity between the plate and screw. However, each of these designs is limited in the strength of the interface between the screw head and the plate due to the reduced surface contact area, and/or reduced mechanical advantage, specific to these features. For example, in a self-tapping design, there is a single ring or collar, within the plate hole that serves as the thread to be tapped by the threads of the screw head. This collar is also designed to be made of an alloy that is weaker than the alloy of the screw, thereby allowing one to tap the other. Inherently, this design cannot be as strong as one in which there are machined threads, providing orders of significantly more surface contact area, and where both screw and plate materials are equally strong, as opposed to mating a weaker material to a stronger material.
The preferred inventions described herein provide an approach for accommodating variable angle locking of a bone screw to an orthopedic plate without necessarily utilizing dissimilar materials, wherein one is weaker than the other.
The preferred variable angle locking design of the present invention enables an orthopedic bone fixation plate to allow a screw to be placed into the bone at various angles, both offset to the plate hole central axis and at radial orientations about the hole circumference, with the same mechanical interface and, therefore, strength, as is found with a fixed angle locking design. That is, with machined threads in the plate that mate with the machined threads on the head of the screw. Mating of machined screw threads on the head of the screw and in the plate is preferred for rigid screw/plate fixation. The preferred invention disclosed herein combines the strength and rigidity of a machined threaded interface between the bone plate and the bone screw, with the capability to position the screw in the bone at angles offset to the plate hole axis and orientations about the circumference of the hole.